After lagging for most of the year, Sonoma County home sales jumped last month to the highest level for September in nine years.

Buyers last month purchased 441 single-family homes, according to The Press Democrat’s monthly housing report compiled by Pacific Union International Vice President Rick Laws. Sales increased 10 percent from a year earlier and amounted to the highest sales for the month since 608 homes were sold in September 2005.

September stronger sales followed sluggish results for August, and agents and brokers acknowledged the difficulty of explaining the latest bump in sales.

“Why September? Who knows?” said Mike Kelley, an agent with Keller Williams in Santa Rosa.

For the first nine months of the year, county home sales remain 7 percent lower than for the same period of 2013. But the total number of homes that changed hands in July through September was nearly the same as a year before.

Some agents suggested the recent sales have been bolstered by buyers seeking homes priced above $700,000, where sales to date have increased nearly 27 percent from a year ago.

“The move up market is back,” said Stephen Liebling, manager of Coldwell Banker’s Sebastopol office. He noted his office has sold more homes this year for more than $1 million.

“We’re really seeing the upper end is very strong,” he said.

In contrast, sales of homes priced below $400,000 have declined to date this year nearly 42 percent from the same period in 2013. Part of that decline is due to a lack of available homes for sale, agents said, and part is due to a rising market that has pushed prices for many homes above $400,000.

September’s median home price rose 5 percent from August to $499,000. The median increased 11 percent from a year ago.

Sonoma County’s median price set a record in August 2005 of $619,000. But prices plunged in a historic housing crash and by February 2009 the median had declined to $305,000.

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— Robert Digitale